Sex question for biologists.

[Mikeisskeptical]

I was recently reading about the origin of sex as one of the biggest mysteries left in evolutionary biology. All of the explanations I have read seem to focus on some long-term or group selection type advantage. ‘For the good of the species’ and that sort of thing. I’m always a little wary of these sorts of explanations and I really don’t understand why an explanation at the level of the individual or the gene fails. I’ve been thinking about it quite a lot and I came up with what seemed to me a reasonable scenario. Perhaps someone could read a very quick summary and tell me if there’s anything wrong with the idea.

The process would begin with the emergence of a sort of ‘proto-male’ in an exclusively ‘female’, asexually reproducing population. This individual learns the trick of copying and pasting some of its genes into another individual, perhaps with something like a virus as an intermediary. It might not initially be 50% of its genes which are transferred, it could be much less and the individual might still gain an advantage in that it can reproduce with multiple partners without having to expend the resources necessary for cell division.

This might seem to the detriment of the ‘impregnated’ individual. After all, they have just suffered a substantial reduction in the number of genes they pass on to each offspring and they still have to expend the energy of actually producing those offspring. However, each of those offspring would also have some chance of inheriting the ‘freeloading’ phenotype.

There would be some advantage to being impregnated in this manner at least some of the time, as your own genes could hitch a ride with the body-hopping genes of your impregnator. By allowing your own reproductive mechanism to be hijacked and reprogrammed to produce another individual’s genes, you also allow the possibility of every other individual in the population having their own machinery redirected to the production of offspring for you.

The benefits of sex seem obvious if you are a male and by extension, equally to a female with a probability of producing male offspring.

There would be competition as to which of the parents contributed the most genes and this would presumably stabilise at about 50% each.

Am I just being really naïve here? I’m untrained in biology, as I’m sure you can tell, so I wouldn’t be at all surprised if what I just said was complete nonsense. If so, thanks for reading it anyway. 

[CalMeacham]

Why assume the existence of a proto-male pasting its genes onto a female?

The advantage of sex, I’ve always understood, is that is provides a way of mixing up characteri9stics to produce variations in offspring. It does so with a much higher probability of variation than random mutation does in asexual reproduction, so sexual reproducers ought to have the edge over asexual reproducers in (relatively) rapidly changing situations. If you have variability among the offspring, there’s a greater likelihood that one of them will be better equipped to deal with the way the environment has changed.

THAT is the immediate payoff – not “the ultimate good of the species”, which natural selection doesn’t recognize, but adaptive radiation that more efficiently exploits niches.

As for how sex itself emerged, I admit that I don’t know, and am not up on the theories about. But I point out that bacterial conjugation also involves transfer of genetic information, and also produces rapid variation ( http://en.wikipedia.org/wiki/Bacterial_conjugation ), and is probably related somehow – either a precursor to sex or an alternative evolutionary strategy.

Isogamy is another mechanism that seems related. ( http://en.wikipedia.org/wiki/Sexual_conjugation ) Note that in neither isogamy nor in conjugation is there one sex “pasting” its characteristics on another – the processes are characterized by exchange of material between two organisms.

[Learjeff]

The best explanation I’ve seen was in “Power, Sex, and Suicide” by Nick Lane, about mitochondria. (When my wife saw me buying that book she gave me the quizzical eye until I mentioned “mitochondria”.)

It’s been a bit, but let’s see if I can pull up the argument. Mitochondria were once independent organisms (similar to archaebacteria) that got subsumed by a host cell. After many generations, much of the mitochondrial DNA migrated to the host’s DNA. Only that DNA that has to be local to the efficient operation and regulation of the individual mitochondrion remained in the mitochondrial DNA (MDNA).

Meanwhile, DNA exchange is common and beneficial, in bacteria as well as eukaryotes. (Simplest reason: so that two mutually beneficial mutations from two different individuals get a chance to mingle.) The exchanged DNA is the host DNA, not the MDNA, but of course, part of the original mitochondrial DNA is now part of the host DNA, and has to be compatible with the reduced MDNA. So, all too often, DNA exchange would result in host DNA that’s incompatible with MTDNA leading to reduced fitness.

Asymmetrical (sexual) reproduction offers a solution to this problem. One parent (the female) provides half of the DNA plus (nearly) all the mitochondria. The other parent provides the other half of the host DNA. If the resulting DNA is incompatible with the mitochondria, the embryo never develops. Lane posits that this accounts for the majority of naturally terminated human pregnancies (usually before they’re even detected), about 1/3 of fertilizations.

In any case, mixing the DNA in only in the child means that both parents survive, and only successful DNA + MTDNA pairings continue to develop.

No doubt I missed a few crucial points.

There’s another benefit to sexual reproduction, I believe, but I haven’t done the math and can’t remember whether this has been supported. It’s been proven (sorry, can’t cite) that the genetic crossover mechanism is proven to be the optimal search algorithm if the only property known about the function is that it’s periodic. Furthermore (and this is the part I’m less sure about, but IIRC) good genes distribute more quickly through a population when parings are between two disjoint classes (i.e., male and female), as compared to when any pairing is allowed (asexual or hermaphroditic).

[Learjeff]

@CalMeacham

The advantage of sex, I’ve always understood, is that is provides a way of mixing up characteri9stics to produce variations in offspring. It does so with a much higher probability of variation than random mutation does in asexual reproduction, so sexual reproducers ought to have the edge over asexual reproducers in (relatively) rapidly changing situations. If you have variability among the offspring, there’s a greater likelihood that one of them will be better equipped to deal with the way the environment has changed.

You’ve answered why gene mixing is beneficial, but not why it needs to be sexual.

[Mikeisskeptical]

@Learjeff

Asymmetrical (sexual) reproduction offers a solution to this problem. One parent (the female) provides half of the DNA plus (nearly) all the mitochondria. The other parent provides the other half of the host DNA. If the resulting DNA is incompatible with the mitochondria, the embryo never develops. Lane posits that this accounts for the majority of naturally terminated human pregnancies (usually before they’re even detected), about 1/3 of fertilizations.

Took me a couple of read-throughs, but that’s a neat solution. Still don’t think my armchair theory is completely refuted. Maybe I need to iron out some of the kinks though. Thanks for the responses.

[Smeghead]

@Learjeff

Lane posits that this accounts for the majority of naturally terminated human pregnancies (usually before they’re even detected), about 1/3 of fertilizations.

I’d have to dig around in the literature to be positive, but I believe this can no longer be considered true. People have looked at spontaneous abortuses, and IIRC, some amount between “lots” and “basically all of them” are caused by chromosomal abnormalities - nondisjunction leading to aneuploidy.